I think that's part of it and you've definitely got me in the right direction.

I think I need 3 separate if functions on c 12,13 and 14 then to add them up in c15. I'm just not sure exactly how to write the last 2.

First one is
=IF($C$10<750,750,$C$10*0.05)

for the next two rows I need to isolate the amounts from 15001 to 35000
and 35,000+